This invention relates to ballast circuits for gas discharge lamps, and more specifically relates to ballast circuits for illumination control systems for gas discharge lamps using a central high frequency power source and which can be dimmed over a wide range for energy conservation purposes.
To conserve energy in lighting applications using gas discharge lamps, it is known that the lamps should be energized from a relatively high frequency source, and that the lamps should be dimmed if their output light is greater than needed under a given situation. For fluorescent lamps, the use of a frequency of about 20 kHz will reduce energy consumption by more than about 20%, as compared to energization at 60 Hz. For high intensity discharge lamps, such as those using mercury vapor, metal halide and sodium, the saving in energy exists but is somewhat less than for a fluorescent lamp.
Energy saved by dimming gas discharge lamps depends on the degree of dimming which is permitted in a given situation. The light output of a lamp is roughly proportional to the power expended. Thus, at 50% light output, only 50% of the full rated power is expended.
Many applications exist where it is acceptable or desirable to decrease the amount of light from a lamp. For example, light in a building might be decreased uniformly or locally in the presence of sunlight coming through a window to maintain a constant or acceptable illumination at a work surface. Thus, during a normal work day, an energy saving of about 50% may be experienced. Light might also be decreased during non-working hours and maintained at a low level for security purposes. Light output might also be decreased, either from local controls or from a generating station during periods of overload on the utility lines.
Energy savings may also be obtained by dimming lamp output when the lamps are new and have a light output much higher at a given input power than at the end of their life. Since a lighted area must be properly illuminated at the end of lamp life, energy can be saved by dimming the lamps when they are new, and then reducing the dimming level as the lamps age. Energy savings of 15% for fluorescent lamps and 20% to 30% for high intensity discharge lamps can be obtained in this fashion.
Copending application Ser. No. 966,604 filed Dec. 5, 1978 in the names of Joel S. Spira, Dennis Capewell and David G. Luchaco and entitled System For Energizing And Dimming Gas Discharge Lamps discloses a central high frequency inverter for energizing a plurality of remote ballasts and associated gas discharge lamps with a substantially continuous periodic output wave form which may or may not be symmetrical. Circuits of any desired sophistication are provided for control of the central inverter and dimming is obtained by varying the amplitude of the voltage and/or current of the inverter output. The connection from the inverter to the ballasts and lamps and remote fixtures is preferably by a novel low-loss transmission line consisting of a pair of spaced conductors which are each insulated by a very thick insulating sheath which minimizes their capacitive and magnetic coupling to one another and to the grounded conduit in which they are located.
Any desired type ballast can be used with the system to perform the basic function of a ballast of limiting lamp current. The ballasts should also satisfy the following criteria:
(1) Preferably, but not necessarily, the ballast should not be destroyed by accidental application of 50 to 60 Hz power.
(2) Preferably, but not necessarily, the ballast should not short the inverter if a single ballast component fails. A short would shut down the inverter until it is located and removed. This problem is especially annoying because the short does not show itself since all lamps are off.
(3) The ballast should exhibit good power factor to the inverter and transmission line.
(4) The ballast should supply a relatively constant filament voltage over the dimming range to avoid damage to lamps. This critera does not apply, of course, to high intensity gas discharge lamps which do not have filaments.
(5) Preferably, the starting voltage must be sufficiently high to strike the lamps under specified service conditions, but starting voltage must not exceed ratings which would damage lamps if the lamps are of the type which could be so damaged.